VMware is a popular virtualization software that allows users to run multiple virtual machines on a single physical server. VMDK (Virtual Machine Disk) files are one of the disk image file formats used by VMware products to represent a virtual hard disk drive (HDD).
VMDK files contain the contents and structure of a virtual hard disk drive that can be accessed by a guest operating system in a VMware virtual machine. These files allow virtual machines to access and store data just like a regular physical machine would on a real hard drive. VMDK is an open standard format and can be used by other virtualization platforms besides VMware.
What is VMware?
VMware is a leading provider of virtualization software and services headquartered in Palo Alto, California. Founded in 1998, VMware pioneered the development of virtual machine technology and the virtualization of computing infrastructure. Their flagship product is VMware vSphere, a server virtualization platform that allows multiple virtual machines with different operating systems and applications to run on the same physical server (About VMware Company).
VMware’s virtualization technology abstracts compute resources from the underlying physical hardware and provisions them to virtual machines. This allows IT organizations to consolidate server sprawl and provide scalable computing resources in a flexible and cost-effective manner. Some key benefits of VMware’s virtualization approach include increased hardware utilization, simplified IT management, faster deployment of apps and services, and built-in availability and security features (VirtualizationOverview).
In addition to vSphere, VMware offers a broad portfolio of products and services for hybrid cloud computing, network virtualization, digital workspaces and more. They are a leading innovator in enterprise-grade virtualization that powers modern IT environments (What is virtualization technology & virtual machine?).
What are VMDK files?
VMDK stands for Virtual Machine Disk Format. It is a file format used by virtualization platforms like VMware to represent virtual hard disk drives (HDDs). VMDK files describe the virtual hard disk drives used by virtual machines (VMs).
According to TechTarget, “VMDK is a file format that allows virtualization products from VMware to emulate hard disk drives. Although the VMDK file format addresses enterprise data center virtualization needs, it can also be used for consumer desktop virtualization.”
VMDK files contain the operating system, applications, and data needed for a VM to function. They allow VMs to mimic the functionality of physical computers. The VMDK format provides portability so VMs can be moved between hosts.
There are two types of VMDK files: sparse and flat. Sparse VMDK files only use disk space as needed and can expand dynamically. Flat VMDK files allocate the full amount of provisioned disk space on creation.
VMDK file structure
VMDK files have a specific internal structure that allows VMware products to mount and interact with virtual hard disk images. According to the VMWare Virtual Disk Format specification (https://github.com/libyal/libvmdk/blob/main/documentation/VMWare%20Virtual%20Disk%20Format%20(VMDK).asciidoc), VMDK files consist of a header, followed by one or more extents (chunks of data).
The VMDK header contains metadata about the virtual disk, including its version, capacity, granularity, etc. After the header are the grain directories and grains themselves. Grains are small fixed-size logical storage units that make up the virtual disk, while grain directories map grains to their logical block addresses.
Within each grain directory is a table with entries for each individual grain, containing the grain’s size, its physical byte offset on disk, a checksum, and a aging timestamp. By dividing up the virtual disk into grains, VMDK files allow for more efficient storage allocation and faster reads/writes when accessing random disk locations (https://medium.com/@avirzayev/the-file-structure-of-a-vm-49759a96c9b1).
In summary, the precise internal layout of VMDK files enables VMware to interact with virtual hard disks at a granular level for better performance and flexibility.
Advantages of VMDK files
VMDK files offer several key benefits that make them a popular virtual disk format (TechTarget, 2022):
- Portability – VMDK files can be easily moved between different virtualization platforms like VMware, VirtualBox, and QEMU. This portability allows virtual machines to be migrated between hosts.
- Performance – The format provides good I/O performance comparable to physical disk drives.
- Snapshots – VMDK supports snapshots to preserve the state of a VM at a point in time. Snapshots facilitate backup and rollback.
- Compact Size – Space-efficient storage is achieved via features like sparse files and compression.
- Scalability – Disk capacity can be expanded online as needed without downtime.
- Data Protection – Integrity checks and other mechanisms guard against data corruption.
In summary, the VMDK format delivers excellent portability, performance, snapshots, and scalability while also providing robust data protection (Nucleus Technologies, 2023). These capabilities make VMDK a highly versatile and beneficial virtual disk format for VMware and other virtualization platforms.
Disadvantages of VMDK Files
While VMDK files offer flexibility and portability for virtual machines, they do come with some downsides and limitations:
VMDK files can become very large, even when the virtual disk isn’t full. According to a post on Reddit, “One of my VM’s has a HUGE VMDK file but the Guest OS isn’t using anywhere close to that much space” (source). This wastes storage space.
There are size limitations on VMDK files. The maximum size is just over 2TB for some versions of VMware products like Workstation and Player (source). For enterprise vSphere, the limit can be as high as 62TB.
Managing a large number of VMDK files for multiple virtual machines can become complex and time-consuming for administrators.
While VMDK files can be portable across VMware products, converting between formats like VMDK and VHD or migrating to other hypervisors can be challenging.
Performance overhead is introduced when accessing virtual disks in VMDK format compared to raw device access.
Finally, recovery from VMDK corruption can be difficult compared to physical disks.
VMware’s Use of VMDK Files
VMware utilizes VMDK files as virtual hard disk drives for its virtual machines. Products like VMware Workstation, vSphere ESXi, and VMware Fusion all use VMDK files as containers to store the operating system, applications, and data for virtual machines.
When a new virtual machine is created in a VMware product, a VMDK file is generated to represent the virtual disk. This VMDK file resides on a physical host machine or on shared storage like a SAN or NAS. The virtual machine then accesses the VMDK as if it were a physical disk drive to read and write data.
VMDK files allow VMware products to provide portability and flexibility for virtual machines. A VMDK from one VMware platform like vSphere can be imported into another platform like VMware Workstation. The virtual disk encapsulated in the VMDK retains all its data.
VMware also supports advanced VMDK features like snapshots, cloning, Storage vMotion, and VMDK monitoring. These capabilities allow efficient virtual machine management and utilization of storage resources.
Overall, VMDK is a core storage technology for VMware’s virtualization platforms. It provides a portable and flexible virtual disk format for virtual machines across many VMware products (Cited from: https://www.techtarget.com/searchitoperations/definition/VMDK-file-virtual-machine-disk-file).
Alternatives to VMDK
While VMDK is a popular virtual disk format used by VMware, there are some alternatives that compete with it for other virtualization platforms:
VHD (Virtual Hard Disk) – Developed by Microsoft as the native virtual disk format for Virtual PC and Hyper-V virtualization products. VHD comes in two formats – fixed size and dynamically expanding. VHD has wider adoption beyond Microsoft products and is used by platforms like Oracle VM VirtualBox. However, VHD does not support some advanced VMDK features like snapshots and sparse disks 1.
VHDX – An enhanced version of VHD that supports larger storage capacity and protection against data corruption. VHDX is optimized for Windows Server 2012 and Hyper-V.
VDI (Virtual Desktop Image) – Used by virtual desktop infrastructure (VDI) products like Citrix XenDesktop. VDI files store both virtual hard disk data and a bootable operating system image. However, VDI does not support features like snapshots or cloning 1.
OVA/OVF (Open Virtualization Format) – An open standard format for packaging and distributing virtual machines. OVF can contain multiple VMDK files and device configurations to describe a virtual machine. It is supported by many virtualization platforms.
Overall, while alternatives like VHD and VDI serve specific virtualization platforms well, VMDK offers a robust feature set that makes it a preferred choice for VMware and other compatible products.
Converting VMDK files
There are several tools and methods for converting virtual machine disk files to and from the VMDK format used by VMware:
VMware provides the VMware Converter tool to convert physical machines and virtual machines from other formats like VHD to VMDK. VMware Converter can also convert VMDK files to platforms like Hyper-V or KVM.
For conversions from VMDK to other formats, third party tools like StarWind V2V Converter or Disk2vhd can be used. They support converting VMDK files to formats like VHD, VHDX, QCOW2, and more. Some of these tools are able to perform offline conversions without needing to boot up or run the VMDK file’s operating system.
The open-source QEMU emulator also provides capabilities to convert disk images between VMDK and other formats. The qemu-img utility can be used from the command line to perform these conversions.
Overall, while VMDK is VMware’s native format, there are various readily available tools to import and export VMDK files to other common virtual disk formats.
Conclusion
In summary, VMDK files are virtual hard disk files used by virtualization platforms like VMware to represent virtual hard drives. VMDK files package virtual hard drive data into files that can be easily moved and managed. VMware heavily relies on VMDK files to provide virtual hard drives to virtual machines created in VMware environments. While alternatives like VHD and VDI exist, VMDK is the native and preferred format for VMware virtual disks. VMDK files can be converted to other formats if needed, providing flexibility. Overall, VMDK files are a critical component of how VMware is able to virtualize storage and provide virtual hard drives to virtual machines.
To directly answer the original question – yes, VMware extensively uses VMDK files as part of its virtualization platform. VMDK files allow VMware to abstract away physical storage into portable virtual hard disk files that can be easily managed and provisioned to virtual machines.
